The gasphase molecular absorption spectrometry method was employed to determine the content of ammonia nitrogen in seawater， and relating factors that might influence the results were discussed. The results showed that when seawater samples from the coast and water samples from the river mouth were adjusted with acid to a neutral pH， there was no need for membrane filtration before direct analysis by the instrument. The storage of the mixed solution of bromate and the solution containing 40% of sodium hydroxide should be no longer than two weeks and the storage temperature should be no higher than 25 ℃， otherwise the solutions should be remade fresh. The collected water samples should not be stored for a long period and they should be analyzed promptly after collection. Using the gasphase molecular absorption spectrometry method， good linear relationships were obtained for ammonia nitrogen in the mass concentration ranges of 0.01~0.40 mg/L and 0.10~2.00 mg/L， respectively， and the coefficient values r were both greater than 0.999. The method detection limit was 0.003 mg/L. RSDs of the actual samples and standards were in the range of 1.0%~1.6%， and the recovery rates were in the range of 94.0%~110%. Compared with the hypobromous acid oxidation method， there was no significant difference in the measurement results for ammonia nitrogen. Because of the large dynamic range for the detection of ammonia nitrogen， the gasphase molecular absorption spectrometry method could be used for samples with high concentrations， which could reduce errors caused by dilution.